In mobile telecommunications systems, there are circumstances where it is desirable for a mobile terminal (such as a telephone, portable computer with communications capabilities, etc.) which is operating at a first frequency in a first network belonging to a first system to transfer to a second network operating at a second frequency (which may belong to a second system, that is, a system using a different type of technology and defined according to a different standard: from this point of view a GSM network belongs to a different system from an UMTS FDD network, for instance). The handover may occur, for example, because the quality of the connection available in the primary network is unacceptably low.
Below, the network in which a mobile terminal is operating before a given handover will be designated the “primary network” and its cells will be referred to as “primary cells”, the network to which the terminal may be handed over will be designated the “target network” and its cells the “target cells”. It should be understood that, in the present document, the word “primary” in the expressions “primary cell” and “primary network” does not denote any particular position of the cell or network in a hierarchy of cells or networks. Moreover, “a cell” may be an omnicell, a sector cell, etc.
Different types of handover may be envisaged, as indicated in Table 1 below:
TABLE 1Primary NetworkTarget NetworkHandover TypeSystem A Frequency ISystem AFrequency JIntra-system, inter-frequency handoverSystem BFrequency JInter-systemhandover
If the primary network is a time division duplex (TDD) network then, even while the mobile terminal is transmitting or receiving data/voice, there are time slots when it is inactive (that is, it is neither sending nor transmitting signals). These time slots can be used to perform measurements on channels operating at other frequencies, thus enabling the terminal to evaluate the performance of candidate target networks.
However, if the primary network is a frequency division duplex (FDD) network, such as a Universal Mobile Telecommunication System (UMTS) FDD network then, when the terminal is active and currently transmitting or receiving data, there are no inactive periods available for performing measurements at other frequencies. So, in this case, the terminal cannot readily evaluate the performance of candidate target networks.
Various techniques have been proposed to enable intra-system inter-frequency handovers, or inter-system handovers, to be performed by terminals operating in primary networks using FDD (such as UMTS FDD networks). These include techniques which use special means to enable measurements to be performed on the candidate target network(s), and so-called “blind handover” techniques.
Techniques Using Measurements on the Target Network:
A first approach which enables measurements to be made on the target network is the “dual receiver” approach in which the radio terminal (e.g. mobile telephone/computer, etc.) is adapted to enable it simultaneously to demodulate two different frequencies.
This approach has a number of disadvantages. Firstly, power consumption of the terminal is increased. Secondly, if the terminal is adapted to operate both in UMTS FDD networks and in GSM 1800 networks then a problem can arise (due to the closeness of the frequencies of the UMTS FDD uplink band and the GSM 1800 downlink band) when the contemplated handover is from an UMTS FDD network to a GSM 1800 network. More specifically, if the frequencies corresponding to the UMTS FDD uplink band and the GSM 1800 downlink band are not perfectly isolated then the dual receiver terminal may not be able to demodulate them both. In such a case another technique would be required in order to enable the terminal to perform measurements on the target network. Finally, the mobile terminal comprises two receivers and, accordingly, requires extra circuitry compared to a standard terminal: which increases its size, cost and complexity.
A second approach which enables the terminal to make measurements on the target network consists in operating the terminal in “compressed mode”. This involves the terminal ceasing reception on its UMTS FDD primary frequency during pattern gaps that are specified in the 3GPP standard (see part TS 25.215, entitled “Physical-layer measurements (FDD)”, of the definition of the 3G standard by the 3GPP). However, compressed mode operation deteriorates link level performance, uplink coverage and the overall capacity of the system.
Furthermore, known handover techniques which involve the making of measurements on the target network delay the triggering of the handover itself. This can lead to the terminal becoming disconnected if the handover was required as a matter of urgency, for example because the terminal had entered a so-called “dead zone” in the primary network.
Blind Handover Techniques
A “beacon pilot” blind handover technique has been proposed in which the target network, which normally operates at a frequency ft, broadcasts a “beacon pilot” at the same frequency fp as the frequency of the primary network. This beacon pilot consists of a pilot channel and a synchronisation channel and enables the mobile terminal to evaluate the propagation loss between itself and the target network.
One disadvantage of the “beacon pilot” approach is that it requires deployment of pilot antennas, increasing the cost of the system infrastructure. Another disadvantage arises in the case of an intra-system, inter-frequency handover between primary and target networks which are UMTS FDD networks operating at adjacent frequencies. In this case the pilot transmission can generate interference on the target network, making its capacity decrease.
Another known blind handover consists in a “direct” blind handover in which a look-up table is held, for example, in the Radio Network Controller (RNC) of the primary network (assuming an UMTS FDD primary network). This look-up table (or “planning table”) indicates, for each primary cell, which target cell should be used in a handover. If the handover is between systems having co-located cells then this blind handover method works reasonably well. However, in the case where the transfer is an inter-system transfer there is no guarantee that the boundaries of the cells of the two systems will be defined in the same locations. If the primary and target cells are not co-located then the quality of the connection available in the target cell will vary depending upon the geographic location of the mobile terminal within the primary cell. Thus, for mobile terminals at certain locations within the primary cell, the target cell specified in the planning table will not be the best one to use.